Image display apparatuses which use display panels whose display is based on a binary illumination system, represented here by plasma display panels (hereinafter referred to as “PDPs”), achieve gray scale by methods such as a method which divides each field of an image into a plurality of subfields, gives each subfield a predetermined luminance weight, and controls whether illumination is ON or OFF in each subfield. For example, in order to display 256 gray scale, each field of an input image signal is divided into 8 subfields which are assigned luminance weights 1, 2, 4, 8, 16, 32, 64, and 128 respectively, and arranged in order of these luminance weights. Then, each bit of an input image signal, which is assumed to be an 8 bit digital signal here, is allocated to the eight weighted subfields in order from the lowest bit, and displayed. That is to say that, illumination is either ON or OFF in each subfield, with 256 gray scale being displayed by arbitrary combinations of the weights. However, in this kind of conventional method of displaying 256 gray scale using eight subfields, flicker components can be observed when the frequency of each frame is around 50 Hz, such as in the PAL (Phase Alternation Line) system. Particularly in large screen display such flicker components are perceived as surface flickers observed over the whole screen, causing a remarkable loss of picture quality. In contrast, flickers are not felt if the field frequency is doubled by signal processing to 100 Hz. Likewise, occurrence of flickers can be controlled by keeping the frequency at the original 50 Hz, and increasing the speed of illumination. This can be achieved by simply doubling the number of subfields to 16 subfields assigned, for example respective luminance weights 1, 2, 4, 8, 16, 32, 64, 128, 1, 2, 4, 8, 16, 32, 64, and 128, and doubling the number of illuminations.
However, in either of these conventional methods the actual emission response speed in the display apparatus must be doubled, meaning the these methods can not always be implemented in plasma display panels in which operation speed is a problem. Furthermore, even if the speed of the display could be increased, a problem still remains that there is no time margin to increase picture quality to enhance performance overall by improving the quality of moving images by making use of the speed response characteristics, improving luminance, and so on.
In particular, in PDPs and the like constraints in cell discharge characteristics and so on mean that there is a limit to how much speed can be increased. As a result, the value of the highest number of subfields that can be set is limited. In addition, in controlling gray scale by controlling subfields, whose possible maximum number is limited, problems arise such as gray level irregularities when moving images are displayed. Therefore, it is not appropriate to perform subfield emission control with the amount of flickers generated being the sole criterion.